This invention relates to a phosphor screen formed by depositing a phosphor layer on a substrate consisting of a fiber plate, and a method of manufacturing said phosphor screen.
An image tube containing a phosphor screen, such as an X-ray image intensifier, is mainly applied in medical uses, although it also used in an industrial X-ray television designed for industrial nondestructive examination.
The above-mentioned X-ray image intensifier is constructed as illustrated, for example, in FIG. 1. An input screen 2 is located, on the input side, within a vacuum envelope 1. An anode 3 and output screen 4 are provided, on the output side, within said glass vacuum envelope 1. A focusing electrode 5 extends along the inner lateral wall of the vacuum envelope 1. The input screen 2 comprises a spherical aluminum substrate 6, an input phosphor layer 7 prepared from CsI and stretched along the output side (concave plane) of said substrate 6, and a photocathode 8 formed on the surface of said phosphor layer 7. The output screen 4 is formed of a substrate 9 and an output phosphor layer 10 mounted on the surface of said substrate 9.
The X-ray image intensifier constructed as described above is operated in the following manner. An X-ray beam penetrating a foreground subject and modulated in accordance with the magnitude of the X-ray transmittance of said foreground subject enters the X-ray image intensifier, to excite the input phosphor layer 7. A light generated by said excitation energizes the photocathode 8, which in turn issues electrons. The released electrons are accelerated by the action of an electron lens comprised of an anode 3 and focusing electrode 5, and focused on the output phosphor layer 10, which in turn irradiates light. The above-mentioned process amplifies the electrons. Thus, a light image decidedly brighter than the light image obtained by the input phosphor layer 7 is released from the output phosphor layer 10.
Japanese Patent Application Disclosure No. 53-24,770 discloses an X-ray image intensifier of the above-mentioned type, which is characterized in that contrast is improved by forming an output phosphor layer on an optical fiber plate. As shown in FIG. 2, an output screen 16 consists of an optical fiber plate 17 and an output phosphor layer 10 deposited on said optical fiber plate 17. The output screen 16 is placed on the output side, within the vacuum envelope 1. The above-mentioned construction of the output screen 16 makes it impossible to directly draw out an image signal from the vacuum envelope, unlike the arrangement in which the optical fiber plate is used as part of the vacuum envelope, and therefore requires the application of a lens system. However, the Japanese disclosure No. 53-24,700 X-ray image intensifier has an advantage in that an accelerating voltage can be impressed in the same manner as in the X-ray image intensifier shown in FIG. 1. Nevertheless, the device proposed in said Japanese patent application disclosure No. 53-24,770 also has drawbacks in that the improvement in the image contrast remains unsatisfactory. The reason for this is given below. FIG. 3 illustrates the manner in which light reflection takes place within an optical fiber. The optical fiber consists of a core 101 and clad 102. Let us assume that n.sub.1 denotes the refractive index of the core 101, n.sub.2 represents the refractive index of the clad 102 and n.sub.0 shows the refractive index of a vacuum. Then, the maximum value of an incident angle .theta..sub.0 with respect to the optical fiber, which is required to assure the transmission of a light through the optical fiber, by repeating total reflection, may be expressed as follows: ##EQU1## For the sake of description, let it be assumed that n.sub.1 equals 1.8 and n.sub.2 equals 1.49. In such a case, the incident angle .theta..sub.0 is determined, from the above equation, to be about 90.degree.. This means that all light rays entering the optical fiber from the region of the vacuum are transmitted through said optical fiber. To confirm this event concretely, the refractive angle .theta..sub.1 of a light ray entering the core 101 at an angle of, e.g., 90.degree. is determined to be 33.7.degree. from the equation, n.sub.1 sin.theta..sub.2 =n.sub.2 sin.theta..sub.0. The critical angle .theta..sub.2 of total reflection at the boundary between the core 101 and clad 102 is determined to be 55.9.degree., from the equation, n.sub.1 sin.theta..sub.2 =n.sub.2 sin.theta..sub.3 (.theta..sub.3 =90.degree.). An incident angle .phi..sub.1 of a light ray having a refractive angle .theta..sub.1 of 33.7.degree. with respect to the boundary between the core 101 and clad 102 is 90.degree.-33.7.degree., which equals 56.3.degree., a value larger than the aforementioned critical angle. Therefore, the light ray is transmitted through the fiber by repeating total reflection, without leaking into the adjacent fiber, and is finally brought to the opposite plane of the fiber to that plane thereof at which the light enters.
When, however, a phosphor layer is deposited over an optical plate, a noticeable change occurs in the above-mentioned process of light transmission. The manner in which the light is transmitted through the fiber plate 17 may now be described, with reference to FIG. 4. The phosphor layer 10 is generally formed by attaching phosphor particles 201 to the surface of the fiber plate 17, by means of a vitreous bonding agent. The fiber plate 17 and phosphor particles 201 are in firm contact with each other, as optically viewed. Since, therefore, light beams emitted from the phosphor particles 201 enter the core 101 of the fiber through said vitreous bonding agent, without being conducted through a free space, some of said light rays are at an incident angle .phi..sub.1 (FIG. 3) exceeding 33.7.degree.. This means that some of the light rays have an incident angle .phi..sub.1 narrower than the critical angle of 55.9.degree.. Light rays having such a small incident angle are transmitted to the adjacent optical fiber. With reference to FIG. 4, a light ray (a) travelling in parallel with the axis of the optical fiber, and a light ray (b) entering a boundary between the core 101 and clad 102 at an incident angle larger than 55.9.degree., transmit through the optical fiber. By way of contrast, a light ray (c) entering said boundary at an angle smaller than 55.9.degree. is successively conducted to the adjacent optical fibers. Consequently, said light ray (c) makes a total reflection at a boundary between the fiber plate 17 and the free space, and is brought back to the phosphor layer 10. Therefore, said light ray (c) will appear to have emitted from phosphor particles different from those from which said light ray (c) originally emitted, thereby making the image contrast low.
Reference may now be made to other phosphor screens in which a phosphor layer is formed on a fiber plate (as set forth in Japanese Utility Model Publication No. 40-19,855 and U.S. Pat. No. 4,264,408). In these phosphor screens, concave indentations are formed on the surface of a fiber plate and phosphor particles are embedded in said concave indentations. However, a phosphor screen having such a fiber plate has a drawback, in that the image contrast is low, since the fiber plate and phosphor particles come into contact with each other oper a broad area. Further, the technique of uniformly embedding the phosphor particles in the concave indentations is accompanied with problems, and lowers the brightness of an image.